1. Field of the Invention
This invention relates to coatings for implantable medical devices, such as stents, and methods for coating the devices.
2. Description of the Background
Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of affected vessels. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location. Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy.
Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient.
One conventional method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
One of the drawbacks and disadvantages associated with the use of such stents has been the relatively rapid depletion of the therapeutic substance from the polymer when the stent is subjected to body fluids. One way to control the rapid rate of release of the therapeutic substance from the coating is by using a topcoat membrane. A topcoat membrane serves as a barrier, retarding the diffusion of the therapeutic substance into the bloodstream.
The present invention discloses various topcoat layers and methods of fabricating the same.